A groundbreaking study published in Science Advances sheds new light on the mysterious origins of free-floating planetary-mass objects (PMOs)—celestial bodies with masses between stars and planets.
Led by Dr. DENG Hongping of the Shanghai Astronomical Observatory of the Chinese Academy of Sciences, an international team of astronomers used advanced simulations to uncover a novel formation process for these enigmatic objects. The research suggests that PMOs can form directly through violent interactions between circumstellar disks in young star clusters.
The Mystery of Rogue Planetary-Mass Objects
PMOs are cosmic nomads, drifting freely through space, unbound to any star. The mass of these objects is less than 13 times that of Jupiter. They are often observed in young star clusters like the Trapezium Cluster in Orion. While their existence is well-documented, their origin has long puzzled scientists. Previous theories proposed that PMOs could be failed stars or planets ejected from their solar systems. However, these models fail to explain the large number of PMOs, their frequent binary pairings, and their synchronized motion with stars within clusters.
"PMOs don't fit neatly into existing categories of stars or planets," said Dr. DENG, corresponding author of the study. "Our simulations show they likely form through a completely different process—one tied to the chaotic dynamics of young star clusters."
A Cosmic Tug-of-War: How Disks Collide to Create PMOs
Using high-resolution hydrodynamic simulations, the researchers recreated close encounters between two circumstellar disks—rotating annuli of gas and dust surrounding young stars. When these disks collide at speeds of 2–3 km/s and distances of 300–400 astronomical units (AU), their gravitational interactions stretch and compress gas into elongated "tidal bridges."
These tidal bridges eventually collapse into dense filaments, which further fragment into compact cores. When these filaments reach a critical mass, they produce PMOs with masses of about ten times that of Jupiter. The simulations also revealed that up to 14% of PMOs form in pairs or triplets, with 7–15 AU separations, explaining the high rate of PMO binaries in some clusters. Frequent disk encounters in dense environments like the Trapezium Cluster could generate hundreds of PMOs, explaining the observed overabundance.
Why PMOs Are Unique
PMOs are distinct in their formation. Unlike ejected planets, they move in sync with the stars in their host clusters and inherit material from the outer regions of circumstellar disks. This results in a unique composition, with PMOs reflecting the metal-poor outskirts of these disks, where heavy elements are scarce. Many PMOs also retain gas disks up to 200 AU in diameter, suggesting the potential for lunar or even planetary formation around these rogue objects.
"This discovery partly reshapes how we view cosmic diversity," said co-author Prof. Lucio Mayer from the University of Zurich, "PMOs may represent a third class of objects, born not from the raw material of star-forming clouds or via planet-building processes, but rather from the gravitational chaos of disk collisions."
Looking Ahead
The team, including researchers from the University of Hong Kong, the Shanghai Astronomical Observatory, the University of California Santa Cruz, and the University of Zurich, plan further studies to explore the chemical makeup and disk structures of PMOs. Upcoming research on PMOs in various clusters will consolidate the theory of their formation and population properties.
